1. Field of the Invention
The present invention relates to a color copying apparatus for forming a color copy image by superimposing a toner image of each color.
2. Description of the Related Art
A conventional color copying apparatus forms a color copy image by superimposing toner images of the three primary colors of cyan (C), magenta (M), and yellow (Y).
An intermediate transfer method is known as the method of forming such a color copy image, comprising the steps of primary transferring in sequence a toner image of each color from a photoreceptor onto a transfer medium (toner image holding medium) rotating at a peripheral speed identical to that of the photoreceptor, to superimpose the toner images on the surface of the transfer medium, and secondary transferring each toner image at one time onto a recording paper (referred to as a "paper" hereinafter).
A color copying apparatus of the intermediate transfer method is provided with a cleaning unit for cleaning the surface of the transfer medium. The transfer medium is cleaned prior to and after the formation of the copy image using the cleaning unit.
The cleaning unit is implemented to be movable between an activating position (at the time of cleaning) and a withdraw position (at the time of copy image formation). This move is generally controlled by the ON/OFF of a solenoid.
During the cleaning prior to the formation of a copy image (referred to as "preliminary cleaning" hereinafter), the cleaning unit is brought to the activating position for at least one rotation of the transfer medium so that excess toner is removed from the whole circumference of the transfer medium. It is generally impossible to completely clean the entire surface of the transfer medium. Toner remains, i.e. wipe residual remains, at an area on the transfer medium confronting the cleaning unit in moving the cleaning unit to the withdraw position at the end of the cleaning.
The circumference of the transfer medium must be longer than the length of the sheet of the maximum size. It is also desirable that the circumference is as short as possible in order to increase the number of copy sheets per unit time.
Accordingly, the circumference of the transfer medium is defined taking into consideration the wipe residual, and is just a little bit longer than the length of the sheet of the maximum size.
The solenoid is driven at a constant timing to move the cleaning unit to the withdraw position.
In an electromagnetic activating unit such as a solenoid, the activating characteristic, i.e. the response speed with respect to a drive control signal, varies according to the temperature. In other words, the response speed becomes slower as the temperature rises due to repetition of ON/OFF.
There was a problem that the copy picture is deteriorated due to wipe residual of the toner images, since the timing of the cleaning unit moving to the withdraw position is delayed depending on temperature. Because a conventional apparatus drives the solenoid at a predetermined constant timing, such a problem was induced.
During preliminary cleaning and the cleaning after the formation of the image (referred to as "subsequent cleaning" hereinafter), the cleaning unit is positioned at the activating state at least for one rotation of the transfer medium, whereby excess toner is removed from the entire circumference of the transfer medium.
Subsequent cleaning is commenced after the secondary transfer. Upon the end of the subsequent cleaning of a predetermined time period, each component of the copying apparatus is returned to the standby state, i.e. the state before the commence of image formation.
The depression of a print key for commencing image formation is generally valid when secondary transfer ends in a color copying apparatus. In other words, the operator can specify the commence of the next image formation after the termination of the secondary transfer and during the subsequent cleaning.
When the print key is pressed during subsequent cleaning in a conventional copying apparatus, cleaning is continued until the elapse of a certain time period defined for subsequent cleaning, followed by a preliminary cleaning for the next image formation, which is also carried out for a predetermined time period.
With both the subsequent cleaning and the preliminary cleaning for the next image formation, the transfer medium will be cleaned continuously for over a time period that exceeds significantly the time of one rotation of the transfer medium, resulting in a problem that the start for the next image formation is delayed.